This invention relates to systems and methods for ion implantation and, more particularly, to methods and apparatus for delivery of low energy ion beams to an ion implantation target, such as a semiconductor wafer. The invention improves the efficiency of low energy ion transport in the mass analysis magnet and in subsequent magnets used in different beam transport architectures.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
In one prior art approach, a high current, broad beam ion implanter employs a high current density ion source, an analyzing magnet to direct a desired species through a resolving slit and an angle corrector magnet to deflect the resulting beam, while rendering the beam parallel and uniform along its width dimension. A ribbon-shaped ion beam is delivered to a target, and the target is moved perpendicular to the long dimension of the ribbon beam to distribute the ion beam over the target.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. However, ion implanters are typically designed for efficient operation at relatively high implant energies and may not function efficiently at the energies required for shallow junction ion implantation. At low implant energies, the current delivered to the wafer is much lower than desired and, in some cases, may be near zero. As a result, long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication costs and is unacceptable to semiconductor manufacturers.
Prior art methods of increasing beam current have often involved mass analyzing and transporting the ion beam at high energies and decelerating the beam to its final low energy at a short distance from the target being implanted. This method results in a mixture of beam energies from ions which neutralize before deceleration is complete. This mixture produces a final energy contaminant at higher energy than the desired primary beam, and this contaminant is implanted at greater depth than the primary beam. Such contamination and its associated distortion of the depth distribution of implanted atoms can produce degradation of the final product or even total failure. It is desirable, therefore, to avoid the deceleration method and to transport the beam at its final intended energy after mass analysis.
Space charge effects can produce rapid divergence of the beam envelope at low energies, impeding transmission and reducing the ultimate beam current delivered to the target. Ion beams require electron clouds to maintain space charge neutralization for good transmission. However, electrons are lost to the walls of the beamline, thereby reducing space charge neutralization when electrons are not supplied to the beam.
Between the polepieces of a magnet, electrons spin around magnetic field lines as they drift through ion beams and may be captured by walls above and below the ion beam. Replacement of electrons is usually accomplished by relying on scattering of beam ions with residual gas atoms, producing ionization. As the beam energy is reduced, the ionization cross-sections are reduced and space charge effects increase. Both effects lead to beam expansion and loss of transmission. Space charge expansion is particularly problematic in magnets, because the beam path through the magnet is relatively long and electrons are not free to move across field lines.
Accordingly, there is a need for improved methods and apparatus for ion beam space charge neutralization in magnets.
According to a first aspect of the invention, an ion implanter is provided. The ion implanter comprises an ion source for generating an ion beam, at least one magnet disposed in the path of the ion beam for deflecting ions in the ion beam, the at least one magnet comprising first and second polepieces spaced apart to define a magnet gap through which the ion beam is transported, an electron source disposed on or in proximity to at least one of the polepieces for producing low energy electrons in the magnet gap, and a target site downstream of the at least one magnet for supporting a target for ion implantation, wherein the ion beam is delivered to the target site. The at least one magnet may comprise one magnet or a plurality of magnets.
In some embodiments, the ion beam comprises a ribbon ion beam having a ribbon beam width and the electron source produces low energy electrons across the ribbon beam width. In other embodiments, the ion beam is scanned so as to produce an effective scan width and the electron source produces low energy electrons across the scan width.
In various embodiments, the electron source may comprise one or more linear electron sources, a one or two-dimensional array of electron emitters, an area electron source or a combination of such sources, with the configuration selected to optimize the transport of electrons to the ion beam.
In some embodiments, the electron source comprises an array of field emitters mounted to at least one of the polepieces and facing the magnet gap. The field emitters produce low energy electrons in the magnet gap.
In some embodiments, the electron source comprises one or more electron-emitting wires disposed in proximity to at least one of the polepieces. The magnet may include a polepiece liner, and the one or more electron-emitting wires may be recessed in the polepiece liner.
According to a further aspect of the invention, a method for transporting an ion beam through a magnet comprises directing the ion beam through a magnet gap between first and second polepieces of a magnet, and supplying low energy electrons to the ion beam being transported through the magnet gap between the first and second polepieces of the magnet.
According to a further aspect of the invention, a magnet assembly is provided for operation with an ion beam. The magnet assembly comprises a magnet disposed in the path of the ion beam and one or more electron sources. The magnet includes first and second polepieces spaced apart to define a magnet gap through which the ion beam is transported. The one or more electron sources are disposed on or in proximity to at least one of the polepieces for producing low energy electrons in the magnet gap.